1. Field of the Invention
This invention relates to an improvement of a scan type optical reader for scanning a subject to be scanned, particularly a bar code pattern, to correctly and easily read information thereof.
2. Related Art Statement
In a scan type optical reader hitherto known, a laser beam of a semiconductor laser emitted through a projecting lens is deflected and projected to a subject to be scanned, and the laser beam reflected by the subject to be scanned is received as a return beam, thereby to read information on the subject to be scanned.
FIG. 1 illustrates a bar code reader 1 as one example of such a conventional scan type optical reader. This bar code reader 1 is a handy type. Within a housing 2 thereof, an optical system is contained. The optical system generally comprises a semiconductor laser 3, a projecting lens 4, a polygon mirror 5, a condenser lens 6, and a photoelectric transferring element 7.
In this bar code reader 1, a bar code information is read in such a manner as that a laser beam P is reflected by the polygon mirror 5 towards a subject 8 to be scanned and deflected to scan the subject 8, and while condensing the laser beam P, as a return beam P', reflected by the subject 8 into the condenser lens 6, the beam P is condensed on the photoelectric transferring element 7.
The laser beam P emitted through the projecting lens 4 is not wave-optically converged into one point. On the contrary, the laser beam P projected by the projecting lens 4 has a beam waist 9 as shown in FIG. 2. The beam diameter 2.phi. of laser beam P projected by the projecting lens 4 at the beam waist 9 is established by F number which is obtained by dividing the distance l between the projecting lens 4 and the beam waist 9 by the effective aperture diameter D of the projecting lens 4. If the proportional coefficients are .alpha. and .lambda., the following relation is obtained for the beam diameter 2.phi. at the beam waist 9 of the laser beam P, the distance l from the projecting lens 4 to the beam waist 9, and the effective aperture diameter D; EQU 2.phi.=(.alpha..multidot..lambda.)l/D (1)
In the above-mentioned relation, the proportional coefficient .alpha. represents an amount relating to an amplitude distribution of the laser beam on the pupil of the projecting lens 4, and the other proportional coefficient .lambda. represents an amount relating to a wave length of the laser beam P.
The beam diameter 2.omega. of the laser beam P increases corresponding to the distance Z from the beam waist 9 like a quadratic function. Therefore, the beam diameter 2.omega. at the subject 8 is established by the beam diameter 2.phi. at the beam waist 9 and the distance Z from the beam waist 9.
The beam diameter 2.omega., as shown in FIG. 3, is directly related to the accuracy of the reading made by the scan type optical reader. If the beam diameter 2.omega. is smaller than the minimum bar distance t of the bar code, the bar code can be correctly read. However, if the beam diameter 2.omega. is larger than the minimum bar distance t and if the laser beam P is projected to a plurality of bars, it becomes difficult to read the bar code easily and correctly.
More specifically, even if the bar code information can be read when the subject 8 to be scanned is within a readable range d which is established by the beam diameter 2.omega.' corresponding to the minimum bar distance t, since the laser beam P spreads with the beam waist 9 placed therebetween, the laser beam P is projected to a plurality of bars when the subject 8 is present far outside of the readable range d. Therefore, it is difficult to read the bar code correctly.
In view of the above, the conventional optical system is suitably designed taking into consideration the distance to the subject 8 to be read, the minimum bar distance of the bar codes, etc.
For example, if a bar code information on the subject 8 placed at a remote place is to be read, the optical system, as shown in FIG. 4, is designed as such that the beam waist 9 of the laser beam P is formed at a remote place. In this way, if the optical system is designed as such that the beam waist 9 is formed at a remote place, although the beam diameter 2.phi. at the beam waist 9 becomes large compared with a case where the beam waist 9 is formed at a near place, the readable range d becomes larger and the spreading angle 2.theta. of the laser beam P becomes smaller, and therefore, the subject 8 placed at a remote place can be correctly read. However, the subject 8 placed at a near place cannot be correctly read. This means that the conventional scan type optical reader can correctly read only a subject which is either placed at a remote place or a near place.
Therefore, in order to correctly read information on the subject 8 to be scanned whether the subject 8 is placed in a near place or a remote place, the optical system is desirably designed as such that the distance from the projecting lens 4 to the beam waist 9 can be changed.
Next, regarding bar code patterns, there are two kinds of patterns; one is the so-called fine specification bar code pattern in which the minimum bar distance (module width) t of the bar code is narrow and the other is the so-called rough specification bar code pattern in which the minimum bar distance t is broad. Hereby, if a consideration is given to a wave-optical image of the laser beam P when the effective aperture diameter D of the projecting lens 4, for example, is formed somewhat large, the diameter 2.phi. (see FIG. 2) of the beam waist 9 becomes small and therefore, the reader becomes suitable for reading information by scanning the fine specification bar code pattern. However, the spreading angle 2.theta. of the laser beam P becomes large and the beam diameter 2.omega. at a distance Z from the beam waist 9 becomes large compared with a case where the effective aperture diameter D is reduced and the beam diameter 2.phi. at the beam waist 9 is increased. As a result, the readable range d becomes narrow.
Therefore, one thought is given to a case where a rough specification bar code pattern is scanned to read information using such designed bar code reader 1. This means that although the bar code can be read by using the beam waist 9 having a larger beam diameter 2.phi. than that of the beam waist 9 for reading a fine specification bar code pattern, the beam diameter 2.phi. of the beam waist 9 for reading a fine specification bar code pattern is used as it is.
In this case, the readable range d is established by the minimum bar distance t of a rough specification bar code pattern and the beam diameter 2.omega.'. Although the readable range d at the time when a rough specification bar code pattern is read, becomes larger than the readable range d at the time when a fine specification bar code pattern is read, the readable range d at the time when the rough specification bar code pattern is read, does not become too large because the spreading angle 2.theta. of the laser beam P is large in the geometrical optics area. Therefore, such an inconvenience comes to be closed up as that when the rough specification bar code is read by using the bar code reader 1 having an optical system which is designed for meeting with the fine specification bar code pattern, the readable range d is relatively narrow.
On the contrary, if the optical system is designed to meet with the rough specification bar code pattern, since the beam diameter 2.phi. becomes large at the beam waist 9, the beam diameter 2.phi. of the beam waist 9 overlaps a plurality of bar codes, the fine specification bar code pattern becomes difficult to read. In this way, the conventional scan type optical reader is inconvenient to correctly read bar codes of different specifications.
Therefore, it is desirable that an optical system of a bar code reader is designed as such that the readable range can be changed so that any bar code having a fine specification bar code pattern or a rough specification bar code pattern can be correctly and easily read.
As apparent from the above-mentioned relation (1), if the effective aperture diameter D of the projecting lens 4 is constant, the beam diameter 2.phi. at the beam waist 9 of the laser beam P is in proportion to the distance l between the projecting lens 4 and the beam waist 9. On the other hand, the beam diameter of the laser beam P is increased in the area near the beam waist 9 like a quadratic function as it departs from the beam waist 9. The change of the beam diameter 2.omega. in the area near the beam waist 9 will be described with reference to FIGS. 5 and 6.
FIG. 5 shows the change of the beam diameter in the area near the beam waist .theta. when the position of the beam waist 9 is changed when the beam diameter 2.omega. is 220 .mu.m at the beam waist 9 in a position 50 mm away from an emitting end 1a (see FIG. 1) of the housing, while FIG. 6 shows the change of the beam diameter in the area near the beam waist 9 when the position of the beam waist 9 is changed when the beam diameter 2.phi. is 190 .mu.m at the beam waist 9 in a position 50 mm away from the emitting end 1a. When the beam diameter 2.phi. is large at the beam waist 9, the inclination of a linear function showing the relation between the position of the beam waist 9 and the beam diameter 2.phi. becomes large. This inclination corresponds to the proportional coefficient .alpha. of the relation (1). It is apparent that if the proportional coefficient .alpha. is small, the information on bar codes placed in a more remote place can be easily and correctly read from the relation between the beam diameter 2.omega.' and the minimum bar distance t.
By the way, the proportional coefficient .alpha. is an amount related to the amplitude distribution of the semiconductor laser 3. As is well known, the semiconductor laser 3 is different in the spreading angle K.sub.1 (see FIG. 7) of the emitting laser beam P in the direction R.sub.1 parallel with a jointing surface 3a and in the direction R.sub.2 vertical to the jointing surface 3a and is different in the amplitude distribution in the direction R.sub.1 extending in parallel with the jointing surface 3a and in the direction R.sub.2 vertical to the jointing surface 3a as shown in FIG. 8.
Therefore, if the semiconductor laser 3 is arranged as such that the elongated axis (R.sub.2 direction) having the large spreading angle K of the laser beam P is parallel with the scanning direction, the proportional coefficient .alpha. becomes small and the beam diameter becomes small in the scanning direction. This means that in order to read both the fine specification bar code placed at near place and the rough specification bar code placed at remote place, contrary to the conventional bar code reader in which the readable range is made large to some extent at the sacrifice of the beam diameter 2.phi. at the beam waist 9, according to the bar code reader in which the beam waist 9 is formed in the near place when the bar code is placed near and the beam waist 9 is formed in the remote place when the bar code is placed in the remote place, it is preferable that the beam diameter 2.phi. is made as small as possible within a range unaffected by unevenness of dust and label on the bar code label at the beam waist 9 without giving a careful thought to the readable range d.
Furthermore, the semiconductor laser 3 is preferably arranged as such that the elongated axis direction where the large spreading angle K is large, is the scanning direction because of the following reason.
That is, if the semiconductor 3 is arranged as such that the elongated direction corresponding to the large spreading angle K.sub.1 is held in parallel with the scanning direction, the change of the laser beam P entering into the projecting lens 4 is relatively small in spite of a presence of irregularity of the spreading angle K due to differences of individual semiconductor lasers 3 as shown in FIG. 9 and therefore, the fluctuation of the beam diameter 2.omega. is small in the scanning direction. However, if the semiconductor laser 3 is arranged as such that the short axis direction where the spreading angle K.sub.1 is small, is in parallel with the scanning direction, the change of the laser beam P entering into the projecting lens 4 becomes relatively large due to irregularity of the spreading angle K.sub.1 of the individual semiconductor laser 3 as shown in FIG. 10 and therefore, the beam diameter is undesirably largely fluctuated in the scanning direction.